Semiconductor packages or encapsulations are typically formed of one or more of metal, plastic, glass, or ceramic materials which are arranged to house one or more semiconductor dies. Such packages may provide protection against impact and corrosion and dissipate heat produced in the die.
In the field of Radio Frequency (RF) electronic devices, such as for example RF power amplifier devices, RF isolation is necessary to reduce the cross-talk of signals from between channels and also RF return currents which may be induced in metal device features. RF isolation also maintains the integrity of amplified signal, and reduces system measurement uncertainties.
RF technologies device packages such as ceramic brazed package constructions or Liquid Crystal Polymer package constructions are known to provide good RF isolation and EM conduction. However these package constructions are expensive to produce.
In general it is known that plastic compound cavity moulded packages provide more cost effective alternatives to ceramics and LCP packages. However, there are a number of challenges in applying plastic mould compound encapsulations to semiconductor devices, most notably minimising and controlling so-called mould flashing (also known as mould creep or bleeding) to keep metallic floating leads, where additional electrical connections such as wire bond will be made free from mould compound. In addition RF semiconductor devices and high voltage MOSFET devices, for example, are known to be high heat dissipating devices which may therefore limit the type of plastic mould compound encapsulations and moulding techniques available. This is due to the potential for damage caused to the encapsulation caused by melting.
With reference to FIG. 1, the leads 10 of the lead frame are said to be floating. The term “floating lead” refers to the situation where prior to moulding the lead 10 is held (or floats) over a base 12 of the device package. The lead is said to “float” because it is not in direct contact with, nor supported by the base 12. During the moulding process the lead will be held at one end by clamping in the moulding machine at a dambar (not illustrated) end of the lead. Following the moulding process it will be mechanically supported by the cured mould compound, as discussed below.
The base 12 of the package is typically formed of a metallic material which functions to mount the semiconductor die (not illustrated in FIG. 1) thereon. The base 12 may also function as a heat sink for the semiconductor device die and may also allow for additional electrical connections by way of appropriate conductive contacts to the semiconductor die, such as for example back contacts on the device die connecting to the base.
As illustrated in FIG. 2, following encapsulation injected mould compound 16 separates the base 12 from the lead 10, and as mentioned above when cured provides mechanical support for the lead 10 on the base 12. As also mentioned moulding techniques may result is some mould flashing occurring. Minimising mould flashing may keep a section of the leads (shown by 13) free from mould compound 16 and as such may allow the leads 10 to be wire bonded by appropriate wire bonding techniques to the semiconductor dies 14 mounted on the base 10, at a later stage of the device packaging process.
A known technique for moulding or encapsulating devices is known as Film (or Foil) Assisted Moulding (FAM). FAM is a so-called transfer moulding technique which uses plastic films in the mould in an attempt prevent liquefied mould compound from reaching certain areas of the device, such as portions of the leads to be wire bonded, during the moulding process. Other known methods for moulding include thermo-compression techniques but they do not involve the use of a film to protect against liquefied mould compound from reaching certain areas of the device.
FIGS. 3 and 4 illustrate, in general terms, a known exemplary arrangement for FAM, where the films 34, 36 is placed over the leads 10, base 12 and one or more semiconductor dies 14 which may be mounted on the base 12. In this example, two films are illustrated, a first film 34 for a top portion of the mould 30, and second film 36 for a bottom portion of the mould 32. During the FAM process the top 30 and bottom 32 portions of the mould are closed around the base 12, leads 10 and device dies 14. Pressure and heat are then applied to the films so that they are sealed around the base 12, leads 10 and device dies 14.
By sealing the films in this way a first void 38 (or cavity) is created around the device dies 14 and by the application of pressure and heat the film is sealed to the base 12 such that the dies 14 are protected from moulding compound which will be introduced later in the moulding process. In FIG. 3, for the purposes clearer illustration the relative size of the first cavity 38, compared to the other features shown has been exaggerated. A second set of cavities (also known as mould cavities) 37 are created at the top and bottom of the leads 10, where the leads overlap the base, and between the leads and the base 12. Later in the moulding process, moulding compound will be injected into the mold cavities 37, thus electrically isolating the leads 10 from the base 12. Once the mould compound has cured, it mechanically supports the leads 10 on the base 12, and the device can be released from the moulding machine.
Once the film is in place the transfer molding process takes place. Generally speaking the moulding process involves liquefied moulding material being forced into closed mould cavities 37 and held under heat and pressure by the top and bottom heat sink clamps 35, 39, as shown in FIG. 4, until the mould material is solidified and cured. The mould is then opened and the encapsulated devices are unloaded, for further device processing, such as mould flash removal and wire bonding.
Film-Assisted Molding offers a number of advantages over other transfer molding techniques and known thermo-compression techniques. These advantages include the easy release of the encapsulated products from the mold, and protection or isolation of surfaces (such as the semiconductor device die and/or leads for later wire bonding) from moulding compound.
However, for this process to be effective the film must be compressed such that it seals to prevent mould flashing of the compound escaping from the mould cavity, especially onto floating leads. Sealing is typically achieved by pressing the film onto the surfaces to be protected by the heat sink clamps 35, 39 from the top and bottom of the device as shown in FIG. 4. Also during the moulding process with the dies already in place it is necessary to keep the device dies isolated from the moulding process and the moulding compound such that the die does not get encapsulated thereby allowing the die to be wire bonded to the leads post-molding.
Due to the nature of the floating leads it can difficult to apply sufficient sealing pressure using heat sink clamps during the moulding process because the leads are floating and not supported by the base 12. The leads are not supported by the base because it is necessary that the leads are electrically isolated from the base. During FAM it is desirable to apply pressure to the film to prevent compound from bleeding. However, if the pressure applied is too great this will deform the lead downwards, resulting in an downward angled (towards the base 12) lead frame surface. The downward deformation makes it difficult to wirebond the lead frame to the device die after the moulding process is complete.
Mould flashing (or polymer bleeds) on semiconductor device leads are a major problem for device manufacturers because they significantly degrade the solderability of the leads and specifically for surface mount devices bleeds can cause misalignments between the printed circuit board (PCB) and can result in the device being tilted on the PCB. Furthermore, the flashes may fall-off during the process of mounting devices on to PCBs due to vibrations occurring during the device mounting process. The loose flash material can fall onto the solderable area of the PCB also causing the device to be partially or not to be soldered which can also result to package tilting on the PCB.
A known solution to the problem of mould flashing (or polymer bleeds) is simply to remove the unwanted polymer material from the leads after the moulding process has been completed. Removal method for polymer bleeds from the leads can be chemical such as a chemical etch, or by mechanically etching. However, such removal methods introduce a further processing step which can in turn add to the time and cost of producing devices.